1. Field of the Invention
The present invention relates to a regenerative direct current motor control which provides regenerative braking by reversing the motor field. More specifically, the present invention is directed to such a control having an improved means for coordinating the operation of the field and armature circuits of the control.
2. Description of the Prior Art
In regenerative direct current motor controls, the braking of the motor necessary for precise speed regulation or other desired operating characteristics is obtained by applying the power generated in the motor during braking back to the active power source for the motor, such as a.c. supply lines.
Regenerative braking is advantageous in that, with proper control of armature current, braking may be done on a permanent basis, whereas resistive braking or other methods, such as plugging, are normally utilizable only in transient conditions or for isolated stops. Braking may also be accomplished very rapidly by regeneration.
Power may be regenerated or returned to the active motor power source by maintaining the same polarity of motor armature voltage while reversing the direction of armature current flow, as in "armature reversing" regenerative motor controls. Motor power may also be regenerated by maintaining motor armature current flow in the same direction while reversing the polarity of the armature voltage. Armature voltage reversal is obtained by reversing the motor field. A motor control employing this technique is termed a "field reversing" regenerative control and is the type to which the present invention is directed. A field reversing control includes a field circuit for controlling and reversing the motor field current and an armature circuit for controlling the motor armature current.
In regenerative braking operation, the reversal of the motor field responsive to a reversal of the motor operative condition error signal reverses the polarity of the motor flux. This reverses the armature counter e.m.f. assuming the direction of motor rotation remains instantaneously the same. The reversed counter e.m.f. biases the thyristors in a thyristor bridge between the a.c. supply lines and the motor armature for current conduction in the same direction as during motoring any time the counter e.m.f. is more negative than the a.c. supply line voltage. This will include a portion of the negative half cycles of the alternating current power, thus permitting the reversal of voltage necessary for regeneration.
A problem in the design and operation of field reversing regenerative motor controls in the past has been coordinating the operation of the field circuit and the armature circuit. The electro-mechanical construction of the motor field winding is such that the inductance, and time constant, of the motor field winding is substantially greater than that of the armature winding. Hence, changes in the magnitude of the armature current can be made more rapidly than changes in the magnitude of the field current. In a typical direct current motor, the ratio of the rapidity of changes in the armature current to changes in the field current may be on the order of 10:1; that is, changes in the armature current may be affected ten times more rapidly than changes in the field current.
During normal operation, it is desirable to allow the armature circuit and the field circuit to operate independently, responsive to changes in the error signal, thereby to take advantage of the rapidity by which changes can be made to the armature current in restoring the operative condition of the motor to desired levels.
However, during the field reversal necessary for regenerative operation, it is imperative to coordinate the operation of the armature circuit with that of the field circuit. The reversal of the motor field winding current occurs during a transient period, the duration of which is established by the motor field winding time constant. In this time interval, the motor field weakens in one direction of current flow, reverses, and increases in the other current flow direction.
During the transient period it is desirable to have small or zero armature current to prevent speed increases as the field weakens and commutator sparking due to armature reaction.
Several techniques for achieving the desired coordination between the field and armature circuit of regenerative motor controls appear in the prior art. For example, U.S. Pat. No. 3,435,316, assigned to the same assignee, discloses the use of logic type switch means operable between a pair of discrete states by the polarity of the motor operative condition error signal and the direction of field winding current flow to remove the input signal to the armature circuit at least during the initial portion of the transient period.
In a typical regenerative motor control, a high gain amplifier is employed at the input of the control so as to permit small reversals of the error signal to initiate regenerative operation and provide close regulation of the operative condition of the motor.
In regenerative motor controls having a switching type of logic, large magnitude signals from the input amplifier may be abruptly applied to the armature circuit, once field reversal has been accomplished. This may drive the armature current immediately to maximum value causing weak field commutator sparking, undesirable suddenness in the commencement of the regenerative braking action, and instability in the operation of the control.
Another way to accomplish the desired coordination between the field and armature circuits of a regenerative motor control is to establish the magnitude of the armature current in accordance with the magnitude of the field current so that as the field current goes to zero during reversal, the armature current is similarly taken to zero. Such a technique also prevents the sudden reapplication of armature current since as the field winding current slowly increases after field reversal, in accordance with the inductive characteristics of the field winding, the magnitude of the armature current will similarly increase. U.S. Pat. No. 3,458,790, also assigned to the same assignee, shows such a control in which the armature current is established by, or "follows" the field current.
However, such a technique may result in an undesirable reduction in the response time of the motor control due to the fact that changes in the magnitude of the armature current are now limited to the much longer time constant of the motor field winding outside the transient period as well as during the transient period.